An internal thread is basically an undercut that does not allow free ejection from the mold. Integrally molded, internal threads are often used for medical connectors. Plastic injection molding of internal threads for certain medical devices presents challenges to designing. In general, injection molding of internal threads, such as a medicine vial cap, can be relatively easy using a slight draft on the core surface to allow the part to be stripped from the core by means of a stripper ring. Provided the draft is suitable and the plastic of proper tensile strength, the part slides off the core easily, the draft reducing the drag quickly and the soft part stripping without deformation. On the other hand, molded-in internal threads on more complex injection molded parts add to the complexity of mold and put constraints on the design. Low production prototype molds can be unscrewed by hand, but most medical device molding is required to be performed automatically. Typically, continuous molding internal threads on plastic medical devices are performed either by stripping, collapsible cores, or unscrewing devices.
Stripped internal threads involves de-mold parts with internal threads by stripping them from the core. This technique is very limited and depends on the plastic used and the thread features. Typically, only thermoplastics with a low modulus of elasticity and a high yield strain are suitable for stripping. Thus, this method is limited at least to certain plastics because during the stripping process, the elastic limit may be exceeded, resulting in defects. Rounded threads are best suited for the stripping method, which further limits design options.
A collapsible threaded core is generally used to mold internal thread forms and freely eject small to medium size parts. Collapsible cores typically result in parting lines appearing on the part due to the segments of the split core. Drawbacks of collapsible cores include flash, plastic build up on the core components requiring cleaning, inconsistent spring pressures, and/or finishing processes for core surfaces. In addition, collapsible cores are more expensive tooling options that are especially prone to wear. These drawbacks make collapsible cores less desirable.
High-quality, large production series internally threaded parts are predominately produced using un-screwing devices (“rotating cores”) integrated with the mold. The majority of all thread forms (especially medium to large) are unscrewed before ejection from the mold. The basic operation is to rotate the core and retract it at the same time and at the same rate as the core is unscrewing from the part. This requires several components working in unison. Usually a rack is used to rotate a gear and an adjustable lead nut times the retraction, which requires an air cylinder or some other means of drive. Rotating cores require bearings to allow rotation of the core during the retraction. The mold typically must have a feature to keep the part from turning while the core is rotating and retracting, e.g., anti rotation lugs, which are cut into the ejection sleeve or into the stripper ring. Thus, a more complex, expensive mold is required.